As the oil and natural gas exploration further developed, high calcium salt content formations with high-temperature or high pressure are encountered more and more frequently in drilling, and it is more difficult to maintain the filtration and rheological properties of the drilling fluid in the drilling process. Consequently, troublesome situations, such as wellbore instability, barite settlement, and jamming of drilling tool, etc., may occur easily. In a high temperature and high calcium salt environment, the additives used to maintain the properties of drilling fluid may have chemical and physical changes such as degradation, crosslinking, desorption, dehydration, and flocculation, etc., which may result in out-of-control of the filtration and rheological properties of the drilling fluid, slow down the drilling speed and increase the operation risks severely. For example, in the Ta-he Oil Field in Xinjiang, high calcium salt and high-pressure high-calcium brine strata are widely distributed and large sections of salt gypsum strata and high-calcium brine contamination are existed in Tazhong, Bamai, Yubei, and peripheral exploration areas. Specifically, for example, in Mabei Well #1, Cambrian high-calcium brine, which has calcium ion content as high as 26,000 mg/L and chloride ion content as high as 179,000 mg/L, was encountered in the drilling process, severe overflow happened, the drilling fluid returned from the bottom of the well flocculated and cured, with poor rheology, and the medium-pressure filter loss was up to 70-80 mL. Since the high-calcium brine infiltrated continuously, it was hardly to maintain and handle the performance of drilling fluid, leading to the abandon of wells. A temperature-tolerant and calcium-tolerant filter loss reducer technology has become a bottle neck in the development of drilling fluids for deep wells and extra-deep wells in the peripheral exploration areas in Xinjiang.
Filter loss reducer is one of the core additives for drilling fluid, and its main function is to promote the drilling fluid to form thin and dense filter cakes with low permeability on the well wall, and thereby reduce filtrate invasion into the strata, decrease the incidence of mud shale hydrated swelling to realize well wall stability. In a case that free high-valence ions (e.g., Ca2+ and Mg2+, etc.) exist, most existing filter loss reducers, including natural modified filter loss reducers and synthesized filter loss reducers, have irreversible strong interactions with the ions, and such interactions are even stronger at high temperatures, resulting in group changes and conformation damages of the molecules of the additives, and even resulting in main-chain breakage and severely compromised effect.
In recent years, a great number of researches have been made on temperature resistance and salinity resistance of filter loss reducers in China and foreign countries. In those researches, 2-acrylamido-2-methylpropane sulfonic acid (AMPS) monomer is mainly used to modify the chain structure of polymeric filter loss reducer, the temperature-tolerant and calcium-tolerant of the product are improved by inducing the two methyl groups of side functional groups and the sulfonic groups of AMPS.
The documents “Synthesis and Performance of AMPS/AM/AN Terpolymer As Filtrate Loss Controller For Drilling Fluids” (Zhonghua Wang, Oil Field Chemistry, 1995, 12(4):367-369), “Synthesis and Evaluation of P(AMPS-IPAM-AM) as a Filtration Reducer” (Zhonghua Wang, Drilling Fluids and Completion Fluids, 2010, 27(2): 10-13) and “Synthesis of P(AMPS-DEAM) Polymer Used as Loss Additive for Anti-calcium Drilling Fluid” (Zhonghua Wang, Fine and Special Chemicals, 2010, 18(4):24-28) disclose an AMPS/AM/AN terpolymer, an AMPS/IPAM/AM terpolymer, and an AMPS/DEAM bipolymer respectively, all of which exhibit good temperature-tolerant and filter loss reducing performance in calcium-containing drilling fluids.
CN201410777862.5 discloses a salinity-tolerant and calcium-tolerant AMPS-AA-AM-DMDAAC copolymerized filter loss reducer for drilling fluid and a preparation method thereof. Owing to the greatly increased molecular weight, the filter loss reducer achieves significant improvement in encapsulation capability, inhibition capability, thermostability, and resistance to salt and calcium contamination, etc.
“Synthesis and Evaluation of High Temperature Resistant and High Calcium Tolerant Filtrate Loss Additive DF-1” (Guancheng Jiang, et al., Oil Field Chemistry, 2015, 32(1):1-6) discloses a drilling fluid filter loss reducer P(AM-AMPS-AA-DMDAAC)/nano-SiO2 synthesized by introducing nano-SiO2, which has high temperature-tolerant, salinity-tolerant, and calcium-tolerant properties.
However, existing filter loss reducers produced domestically can effectively control the filter loss only if the temperature is lower than 150° C. and the Ca2+ content is lower than 1.4×104 mg/L. There is no domestic filter loss reducer that has higher temperature-tolerant capability under the condition of higher calcium ion contents; moreover, a lack of control on molecular weight, normally over high, leads to a static structural force of drilling fluid, which has no benefit to rheology control of n high-temperature and high-density drilling fluid.
Hence, there is an urgent need for a calcium-tolerant and temperature-tolerant filter loss reducer for drilling fluid.